1. Field of the Invention
The present invention is related to a system and method for monitoring a dropped amount of liquid crystal.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) includes upper and lower display panels with electrodes formed thereon and a liquid crystal (LC) layer disposed between the upper and lower display panels. The upper and lower display panels are coupled with each other by a sealant and are supported by a spacer. As the LC molecules of the LC layer arrange themselves according to a voltage applied to the electrodes, an image is displayed by adjusting the voltage to control the transmittance of light through the LC layer.
As the LCD panels become larger, the method for forming the LC layer during the manufacturing of the LCD is evolving from the conventional process of injecting liquid crystal in a vacuum to dropping the liquid crystals on the panel.
The process of dropping an LC can be largely divided into three stages. In the first stage, a sealant is applied on a lower display panel to define a closed-loop active area. in the second stage, LC is dropped on the active area of the lower display panel. Then, the lower display panel is assembled with the upper display panel in vacuum and the sealant is hardened.
From a productivity standpoint, the process of dropping the LC is more advantageous than the LC injection method that is currently widely used.
However, since an assembly process is immediately performed in vacuum after the LC has been dropped, a defect may form in an active unfilled area (AUA) if the amount of the dropped LC is insufficient. If the amount of dropped LC is excessive, a defect may form as gaps near an edge.
The inner pressure in the gap between the assembled panels depends on the amount of the dropped LC. Accordingly, the LC should be dropped with an accuracy of a 1% error range to avoid defects. In order to accurately control the total amount of the dropped LC, the amount of the LC should be accurately monitored while the LC is being dropped. Furthermore, in order to maintain the inner pressure of the assembled panels at a predetermined level, the amount of the dropped LC is determined relative to the height of a column spacer. If the level of the dropped LC is higher than the column spacer, i.e., in the case that the amount of the dropped LC is more than a predetermined amount, a mura may be caused. On the other hand, if the amount of the dropped LC is less than the predetermined amount, an air bubble may form in the LC layer.
Since the exact amount of LC in a drop may vary, it is necessary to measure the amount of the LC being dropped on the active area at a regular time interval even if an LC dropper is accurately set up.
Currently, in order to accurately detect the amount of the LC being dropped on the active area, test amounts of LC is dropped on an electronic balance. In this case, a change in the amount of the LC being dropped on the active area can be minimized by shortening the measuring cycle.
Because the amount of the test LC is very small (e.g., about 1 to 4 mg), the test LC must be dropped more than 10 times in order to obtain sufficient accuracy with this test-amount method. An undesirable consequence of this repeated testing is that a substantial amount of the test LC is spent, increasing the test cost. Another problem with this test-amount method is that the weight and the deviation of the dropping LC are not the actual weight and deviation of one LC drop but an average value based on the weight and variation over several decades of the LC drops. Therefore, the accuracy is problematically deteriorated.
A method for accurately and cost-effectively measuring the amount of LC that is dropped onto a display panel is desired.